Most organo-sulfur compounds possess a strong and unpleasant odor. Thus, gases and liquids, which contain even a very small amount of these compounds, have a bad smell. For some applications, as for city gas, this is a desired side effect to avoid hazardous situations, but in most cases, sulfur compounds are troublesome impurities that need to be removed. Owing to this problem, the technology-of removing these substances is conventionally termed as “sweetening” or deodorization. These sulfur-contaminated compounds are also corrosive, causing damage to technological equipment and transportation systems. Further, practically all sulfur-contaminated compounds are irreversible poisons for many catalysts used in chemical processes. Therefore, such commercially important processes as natural gas steam reforming, individual hydrocarbons and petroleum distillate isomerisation, hydrogenation, etc. require practically complete removal of the many sulfur compounds from the process feed before catalysis. Finally, it should be mentioned, that the full oxidation of the organic sulfur compounds leads to sulfur dioxide and sulfur trioxide, whose formation needs to be minimized for ecological reasons.
Removal of sulfur containing compounds is normally done in two steps. In a first stage, the amine treatment removes hydrogen sulfide from the system. Some mercaptans, part of carbon oxysulfide and of carbon dioxide may also be removed in this step. This process is related to absorption. The second step is an adsorption of organic sulfur compounds, especially mercaptans, sulfides, thiophenes, thiophanes and disulfides.
Adsorption of sulfur-contaminated compounds is the most common method for removal of these sulfur compounds, because of the high performance and relatively low capital and operational costs. Numerous processes and adsorbents have been developed for the removal of organic sulfur compounds and hydrogen sulfide, carbon oxysulfide and carbon disulfide, from gases and liquids.
The most widely used physical adsorbents for these sulfur compounds are synthetic zeolites or molecular sieves. For example, U.S. Pat. No. 2,882,243 and U.S. Pat. No. 2,882,244 disclose an enhanced adsorption capacity of molecular sieves NaA, CaA, and MgA for hydrogen sulfide at ambient temperatures. U.S. Pat. No. 3,760,029 discloses the use of synthetic faujasites as an adsorbent for dimethyl disulfide removal from n-alkanes. U.S. Pat. No. 3,816,975, U.S. Pat. No. 4,540,842 and U.S. Pat. No. 4,795,545 disclose the use of standard molecular sieve 13X as a sulfur adsorbent for the purification of liquid hydrocarbon feedstocks. For removal of carbonyl sulfide, mercaptans, and other sulfur compounds from liquid n-alkanes, U.S. Pat. No. 4,098,684 discloses the use of combined beds of molecular sieves 13X and 4A. EP 0 781 832 discloses zeolites of types A, X, Y and MFI as adsorbents for hydrogen sulfide and tetrahydrothiophene in natural gas feed streams.
Regeneration of these molecular sieves is possible at elevated temperatures. To facilitate regeneration of the molecular sieves by removing the sulfur compounds adsorbed, the use of cation exchanged forms of zeolite types A, X, Y have been proposed due to their catalytic activity in the reduction or oxidation reaction of sulfur compounds at the regeneration stage. For instance, U.S. Pat. No. 4,358,297 discloses regeneration of the adsorbent using hydrogen or a hydrogen-contaminated stream at elevated temperatures, 200-650° C., resulting in conversion of the organo-sulfur compounds to hydrogen sulfide. U.S. Pat. No. 5,843,300 discloses a regenerable adsorbent for gasoline purification that comprised a standard zeolite X impregnated with up to 1% by weight zero valent platinum or palladium. This noble metal component provides hydrogenation of the adsorbed organic sulfur compounds on the course of the adsorbent regeneration. However, the introduction of noble metals into the adsorbent composition substantially increases the cost of the adsorbent.
During adsorption and especially during desorption, the mercaptans undergo chemical transformations. The mercaptans may form sulfides, disulfides or alkenes and hydrogen sulfide according equation (1), (2) and (3). This phenomenon is described in detail in K.-H. Bergk, F. Wolf, Z. Chem. 1974, 14(9), 344-349.

The alkenes are unstable under the regeneration conditions and tend to oligomerise and ultimately will lead to coke formation in the zeolite pores. L. N. Gimadeev et al., Gazov Prom-st 1985, 9, p. 34 describe that at a regeneration temperature of 350° C. after a few adsorption cycles, the coke formation will reduce the adsorption capacity dramatically.
Ziolek et al., Pr. Nauk. Inst. Chem. Technol. Nafty Weg 1996, 55(8), 67-73 discuss the influence of Bronsted acid, Lewis acid and/or basic sites on the zeolite surface to the catalytic decomposition of mercaptans during regeneration. However, they fail to teach on how to deactivate such centers on a zeolite.
All these molecular sieve adsorbents can work at ambient temperature and have a substantial capacity for removal of sulfur compounds at relatively high concentrations. While all these products have been useful for gas and liquid stream purification of sulfur-contaminated compounds, they need special arrangements to get full regeneration.
It is a main aspect of the present invention to enhance the lifetime of zeolitic adsorbents, in particular by providing improved adsorbents and processes which do not have the disadvantages of the regeneration mentioned above. Accordingly, it is an aspect of the invention to provide an adsorbent and a process for purification of sulfur-contaminated feed streams with improved regeneration capabilities.
It is a further aspect of the invention to provide a low cost adsorbent for sulfur compounds.
It is a further aspect of the invention to provide an improved process for regeneration of the molecular sieve.
It is still a further aspect of the invention to disclose an adsorbent with capability to purify feed streams of practically all organo-sulfur compounds, including thiols (mercaptans), sulfides, disulfides, thiophenes, thiophanes, etc. as well as hydrogen sulfide, carbon oxysulfide, and carbon disulfide, individually or in combination thereof.
These and further aspects of the invention will be apparent from the description of the invention, and in particular of the preferred embodiments thereof.